The ultimate performance of 5G depends on many technical and regulatory factors, including sufficient spectrum, the presence of affordable equipment and access to sufficient cell sites. One factor that doesn’t get the amount of attention it deserves, however, is the technical approach to how networks coexist so they can operate in most efficient way possible. If regulators and governments don’t get this right, 5G performance will likely fall short of expectations.
The technical details of networks have a major impact on coverage and performance. In the 5G world, we will see the first major rollout of Time Division Duplex (TDD) cellular networks in many countries because of the specifications for the use of the 3.5 GHz spectrum range in the 5G New Radio (NR) standard.
Unlike many of today’s networks, where download and upload traffic generally have the same amount of capacity in separate frequency bands (Frequency Division Duplex – FDD), TDD affords more flexibility. It does this by using the same spectrum band for upload and download traffic, separated in terms of time (think of it as “talking” then “listening” at very precise time intervals).
The TDD approach allows for tailored network performance, as the amount and timing of talking and listening can be adjusted based on the application. Imagine downloading a book to read one page at a time – you may be able to prioritise downloading and not be terribly bothered by having a lot of uploading capability close to it. If, however, you are remotely controlling a robot, you would want your commands and the information about the results of those commands be delivered back to you quickly. In optimising a network for either approach you would configure the amount and timing of uploading and downloading very differently.
The flexibility that TDD offers, however, has to be reconciled with the need for adjacent networks to coexist peacefully. A key way to support this coexistence is synchronisation. That is, the synchronisation of technical parameters in adjacent TDD networks to make sure they send and receive data from mobile devices at the same time in order to avoid interference.
While there are alternative approaches, such as adding guard bands – either in frequency or geography – so the systems are not directly adjacent or adding filters to the equipment, they have drawbacks severe enough to make them unviable. For example, CEPT, the coordinating body for European telecommunications organisations, indicates that the required physical separation distance between two unsynchronised networks. It is up to 60km for those operating on the same frequency channels and up to 14km for those operating on adjacent channels. This means either uncovered area or unused spectrum in between these networks.
So, what must be done to get TDD synchronisation, and by extension 5G, right? The GSMA has developed new guidelines and recommendations to help. The recommendations focus on the way in which the network is configured with respect to the timing of Download (D), Special slot (S) and Upload (U) elements in each period of time (the frame). The frame format the GSMA supports is delineated as DDDSU (3 download followed by one special slot followed by one upload). As described in detail in the guidelines document (link below), this approach provides a good compromise between download and upload speeds with a low latency, based on current requirements.
To support a harmonised approach for network and consumer equipment, in an ideal world, all of the 3.5 GHz range would be available for 5G globally. Unfortunately, that isn’t the case. In some countries, this spectrum is already used by LTE or WiMax technologies. While the GSMA recommends upgrading these networks to 5G NR or migrating them to a different band as soon as possible to make more efficient use of the spectrum, this may take some time.
In the meantime, coexistence between LTE and 5G NR networks within a country or a common coverage area can be achieved by selecting a compatible frame structure for those national networks, some possibilities are discussed in the guidelines document. Where LTE networks are very localised which is often the case, then alternatives coexistence approaches should be considered.
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